561 research outputs found
Photometric stability analysis of the Exoplanet Characterisation Observatory
Photometric stability is a key requirement for time-resolved spectroscopic
observations of transiting extrasolar planets. In the context of the Exoplanet
Characterisation Observatory (EChO) mission design, we here present and
investigate means of translating spacecraft pointing instabilities as well as
temperature fluctuation of its optical chain into an overall error budget of
the exoplanetary spectrum to be retrieved. Given the instrument specifications
as of date, we investigate the magnitudes of these photometric instabilities in
the context of simulated observations of the exoplanet HD189733b secondary
eclipse.Comment: submitted to MNRA
Herschel observations of the Sgr B2 cores: Hydrides, warm CO, and cold dust
Sagittarius B2 (Sgr B2) is one of the most massive and luminous star-forming
regions in the Galaxy and shows chemical and physical conditions similar to
those in distant extragalactic starbursts. We present large-scale far-IR/submm
photometric images and spectroscopic maps taken with the PACS and SPIRE
instruments onboard Herschel. The spectra towards the Sgr B2 star-forming
cores, B2(M) and B2(N), are characterized by strong CO line emission, emission
lines from high-density tracers (HCN, HCO+, and H2S), [N II] 205 um emission
from ionized gas, and absorption lines from hydride molecules (OH+, H2O+, H2O,
CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO
suggest the presence of two gas temperature components: an extended warm
component, which is associated with the extended envelope, and a hotter
component, which is seen towards the B2(M) and B2(N) cores. As observed in
other Galactic Center clouds, the gas temperatures are significantly higher
than the dust temperatures inferred from photometric images. We determined
far-IR and total dust masses in the cores. Non-local thermodynamic equilibrium
models of the CO excitation were used to constrain the averaged gas density in
the cores. A uniform luminosity ratio is measured along the extended envelope,
suggesting that the same mechanism dominates the heating of the molecular gas
at large scales. The detection of high-density molecular tracers and of strong
[N II] 205 um line emission towards the cores suggests that their morphology
must be clumpy to allow UV radiation to escape from the inner HII regions.
Together with shocks, the strong UV radiation field is likely responsible for
the heating of the hot CO component. At larger scales, photodissociation
regions models can explain both the observed CO line ratios and the uniform
L(CO)/LFIR luminosity ratios
The ISO LWS high resolution spectral survey towards Sagittarius B2
A full spectral survey was carried out towards the Giant Molecular Cloud
complex, Sagittarius B2 (Sgr B2), using the ISO Long Wavelength Spectrometer
Fabry-Perot mode. This provided complete wavelength coverage in the range
47-196 um (6.38-1.53 THz) with a spectral resolution of 30-40 km/s. This is an
unique dataset covering wavelengths inaccessible from the ground. It is an
extremely important region of the spectrum as it contains both the peak of the
thermal emission from dust, and crucial spectral lines of key atomic (OI, CII,
OIII, NII and NIII) and molecular species (NH3, NH2, NH, H2O, OH, H3O+, CH,
CH2, C3, HF and H2D+). In total, 95 spectral lines have been identified and 11
features with absorption depth greater than 3 sigma remain unassigned. Most of
the molecular lines are seen in absorption against the strong continuum,
whereas the atomic and ionic lines appear in emission (except for absorption in
the OI 63 um and CII 158 um lines). Sgr B2 is located close to the Galactic
Centre and so many of the features also show a broad absorption profile due to
material located along the line of sight. A full description of the survey
dataset is given with an overview of each detected species and final line lists
for both assigned and unassigned features.Comment: Accepted for publication in MNRA
OH/IR stars and their superwinds as observed by the Herschel Space Observatory
Aim : In order to study the history of mass loss in extreme OH/IR stars, we
observed a number of these objects using CO as a tracer of the density and
temperature structure of their circumstellar envelopes.
Method : Combining CO observations from the Herschel Space Observatory with
those from the ground, we trace mass loss rates as a function of radius in five
extreme OH/IR stars. Using radiative transfer modelling, we modelled the dusty
envelope as well as the CO emission. The high-rotational transitions of CO
indicate that they originate in a dense superwind region close to the star
while the lower transitions tend to come from a more tenuous outer wind which
is a result of the mass loss since the early AGB phase.
Result : The models of the circumstellar envelopes around these stars suggest
that they have entered a superwind phase in the past 200 - 500 years. The low
18O/17O (~ 0.1 compared to the solar abundance ratio of ~ 5) and 12C/13C (3-30
cf. the solar value of 89) ratios derived from our study support the idea that
these objects have undergone hot-bottom burning and hence that they are massive
M >= 5 solar-mass AGB stars.Comment: 10 pages with 11 figures Accepted for publication by Astronomy &
Astrophysic
Oxygen isotopic ratios in galactic clouds along the line of sight towards Sagittarius B2
As an independent check on previous measurements of the isotopic abundance of
oxygen through the Galaxy, we present a detailed analysis of the ground state
rotational lines of 16OH and 18OH in absorption towards the giant molecular
cloud complex, Sagittarius B2. We have modelled the line shapes to separate the
contribution of several galactic clouds along the line of sight and calculate
16OH/18OH ratios for each of these features. The best fitting values are in the
range 320-540, consistent with the previous measurements in the Galactic Disk
but slightly higher than the standard ratio in the Galactic Centre. They do not
show clear evidence for a gradient in the isotopic ratio with galactocentric
distance. The individual 16OH column densities relative to water give ratios of
[H2O/OH]=0.6-1.2, similar in magnitude to galactic clouds in the sight lines
towards W51 and W49. A comparison with CH indicates [OH/CH] ratios higher than
has been previously observed in diffuse clouds. We estimate OH abundances of
10^-7 - 10^-6 in the line of sight features.Comment: 10 pages, 6 figures, accepted for publication in A&
The dust mass in Cassiopeia A from a spatially resolved Herschel analysis
Theoretical models predict that core-collapse supernovae (CCSNe) can be efficient dust producers (0.1â1.0 Mâ), potentially accounting for most of the dust production in the early Universe. Observational evidence for this dust production efficiency is however currently limited to only a few CCSN remnants (e.g. SN 1987A, Crab nebula). In this paper, we revisit the dust mass produced in Cassiopeia A (Cas A), a âŒ330-yr old O-rich Galactic supernova remnant (SNR) embedded in a dense interstellar foreground and background. We present the first spatially resolved analysis of Cas A based on Spitzer and Herschel infrared and submillimetre data at a common resolution of âŒ0.6 arcmin for this 5 arcmin diameter remnant following a careful removal of contaminating line emission and synchrotron radiation. We fit the dust continuum from 17 to 500 ÎŒm with a four-component interstellar medium and supernova (SN) dust model. We find a concentration of cold dust in the unshocked ejecta of Cas A and derive a mass of 0.3â0.5 Mâ of silicate grains freshly produced in the SNR, with a lower limit of â„0.1â0.2 Mâ. For a mixture of 50 per cent of silicate-type grains and 50 per cent of carbonaceous grains, we derive a total SN dust mass between 0.4 and 0.6 Mâ. These dust mass estimates are higher than from most previous studies of Cas A and support the scenario of SN-dominated dust production at high redshifts. We furthermore derive an interstellar extinction map for the field around Cas A which towards Cas A gives average values of AV = 6â8 mag, up to a maximum of AV = 15 mag
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